Owing to the mode of operation of a hydraulic wheel brake, the brake piston is retracted somewhat into the brake caliper housing by the piston sealing ring when the service brake is not actuated. In addition, the brake disk may be deflected somewhat out of its rest position and the brake piston may be pushed further back into the brake caliper housing during dynamic driving maneuvers (rapid cornering, driving over irregularities). Overall, this increases the volume required to reapply the brake piston to the brake pads. Since the delivery rate of the hydraulic actuator of an ESP system (Electronic Stability Program) is limited, a hydraulic volume is first of all required in the brake caliper housing in order to push the brake piston in the direction of the pads. Only then is a pressure and force buildup possible. During this process, there is a loss of time between the demand for braking and the provision of a braking force.
The prior art includes DE 10 2006 029 979 A1, for example. This describes a method for operating a brake system of a motor vehicle having at least one operating mode involving pre-filling for at least partially overcoming a release clearance of one or more wheel brakes before an expected actuation of a brake pedal. Here, the release clearance is overcome by the hydraulic unit, i.e. by the hydraulic service braking system.
The prior art furthermore includes DE 101 11 076 B4, for example. This describes a brake disk wiper, wherein the brake actuators are controlled in such a way when an activation condition is met that the brake pads are placed against the brake disk in such a way that no deceleration that is perceptible to the driver or a deceleration which is virtually imperceptible to the driver is set. In this case, the brake actuators are the actuators of a hydraulic service brake system.
However, the buildup of a hydraulic pressure by means of the hydraulic service brake system to place the brake pads in contact gives rise, when there is actual contact, to a link, i.e. coupling, between the actuator and the brake pads. As a result, unwanted braking forces due to the prevailing hydraulic pressure can arise. Moreover, unwanted braking pulses and periodic noises, which negatively affect the perception of comfort of the driver, can occur owing to runout in the brake disk, for example.
In addition, known hydraulic actuators must first of all draw in brake fluid in order to be able to build up pressure. At low temperatures, the viscosity of the brake fluid increases, making intake more difficult. Pressure is provided only with a delay.
It is therefore the object of the present disclosure to provide a braking device and a method for controlling the braking device, by means of which pressure buildup dynamics can be improved, i.e. by means of which a loss of time between the demand for braking and the provision of braking force is minimized. However, the buildup of a braking force and an interaction between the brake disk and the brake actuator should be substantially avoided until there is an actual demand for a braking force (i.e. until the demand for braking).
This object is achieved by the features of the disclosure. Developments of the disclosure are provided by additional features of the descriptions, drawings, and claims.
For this purpose, a braking device is provided for a motor vehicle. This braking device comprises at least one brake, at least one brake pad and a brake disk, wherein the brake pad can be placed against the brake disk in such a way that no braking force or only a small braking force acting on the motor vehicle is built up. According to the disclosure, it is envisaged that the brake pad can be placed against the brake disk by means of an elastic element.
This is understood to mean that a brake actuator places the brake pad against the brake disk by means of the elastic element. During this process, a movement and/or a force is transmitted to the elastic element by actuation of the brake actuator. In particular, the force and/or movement is orthogonal with respect to the brake disk. During this process, a movement or elastic force in the elastic element is also produced.
A brake piston can furthermore be provided, said piston being positioned between an actuator of the brake and the brake pad. Here, the brake piston converts a force or movement provided by the actuator, for example, into a clamping force between the brake pad and the brake disk.
As regards its elasticity, the elastic element is designed to be sufficiently powerful to ensure that the elastic force resulting from the actuator movement in the elastic element is sufficient to move the brake piston in the direction of the brake disk. At the same time, however, the elastic element is designed in such a way, as regards its elasticity, that, when the brake pads are placed in contact, the clamping force resulting from the elastic force transmitted to the brake disk is so small that no braking force or only a small braking force is produced. Such a small braking force thus leads to no deceleration or to a deceleration which is imperceptible or virtually imperceptible to the driver. The brake pads are thus placed against the brake disk in such a way by the elastic element that the driver does not perceive actual contact of the brake pads.
By means of the device presented, a large proportion of the required hydraulic volume is already moved in advance of the actual braking operation. It is thereby also possible largely to avoid a delay in the provision of braking force due to the change in viscosity of the brake fluid since, as a result, even small volume flows in the intake channel of the pump lead directly to provision of a braking force at low temperatures.
It should furthermore be mentioned that a disk is generally not completely free of disk runout. However, the unbalance of the brake disk arising in the case of braking runout is, by virtue of the elastic element, advantageously not transmitted to the brake system; on the contrary, the elastic element enables the brake piston or the brake pads to follow the disk geometry. The braking torques which arise in this case are very small.
In addition to use for providing emergency braking and use in the region of autonomous driving, a device of this kind can also be used as a brake disk wiper in order, for example, to wipe an unwanted film, in particular water, from the brake disk without the need for the driver to intervene to achieve this or feeling this due to a deceleration of the vehicle.
It is advantageous if the braking device has a service brake and an automated parking brake. This should be taken to mean that the at least one brake is configured as a service brake and as an automated parking brake. The braking device therefore has two brake systems, in particular two different brake actuators. In particular, the service brake is configured as a hydraulic device with a brake fluid as an operating medium. In particular, the service brake should be regarded as a standard vehicle brake. Here, the service brake can advantageously be configured as an ESP system. In this case, an ESP pump forms the brake actuator of the service brake. As an alternative to an ESP pump as a brake actuator, pressure can also be provided by an i-booster or some other pressure-supplying system (EHB, plunger system).
The automated parking brake, also referred to as an automated (or automatic) immobilizing brake, is advantageously configured as an electromechanical device, having an electric motor as a brake actuator for the automated parking brake, a spindle and a spindle nut. The service brake and the parking brake can be combined in one housing. Furthermore, both brakes can also use one and the same brake piston, brake pads and brake disk. Coupling and assistance between the two brakes is advantageously possible, e.g. the hydraulic service brake can be used to assist, by means of hydraulic pressure, application or, in particular, further tightening of the electromechanical parking brake.
It is advantageous if, on the basis of actuation of the parking brake, the brake pad is placed against the brake disk by means of an elastic element. To place the brake pad against the brake disk, the brake piston is moved, as already described. Provision is made here for a brake actuator of the parking brake to move the brake piston by means of the elastic element and thus to place the brake pad against the brake disk.
In this case, use is advantageously made of the possibility of setting a particular and lasting deflection of the parking brake, said deflection serving as a basis for the use of the elastic element: the defined deflection of the parking brake taking into account a possible deformation of the elastic element eliminates a release clearance between the brake pad and the brake disk, wherein the elastic element furthermore allows partial decoupling of the actuator from the brake disk.
As an alternative, it would also be possible to supply a hydraulic pressure which presses the brake pistons “gently” against the brake pads. However, it is disadvantageous here that low pressures can be set only with difficulty. This is most readily possible if the backpressure of the hydraulic actuator and of the downstream components thereof is exploited. In this case, a correspondingly large volume is displaced, while the valves are not closed. The brake pistons are pressed against the brake pads by the backpressure. If the hydraulic actuator is switched off, the brake piston returns to its initial position again. It is not possible in this way to implement prolonged contact between the brake pads and the brake disk in a manner which is favorable in terms of energy.
“Trapping” a low pressure also has disadvantages. The trapped pressure forms a resistance to the brake piston being forced back into the brake caliper housing. Since the brake disk is never 100% free from axial runout, disk runout repeatedly leads to pressure and hence also clamping-force spikes, which lead primarily to increased friction and hence also to increased wear. Moreover, the friction also imposes thermal stress on the disk and the pads.
The use of the parking brake overcomes the disadvantages which have been described of a possible use of the service brake for acting on the elastic element to place the brake pad against the brake disk.
The parking brake advantageously places the brake pad against the brake disk by means of the elastic element. This should be taken to mean that the parking brake is activated and undergoes a change, e.g. a change in length. On the basis of this change, it is possible for the parking brake to produce a change, e.g. change in length, in the elastic element. By virtue of the characteristic of the elastic element, said element likewise transmits such a change, if only to a limited extent, to the brake pad and places the latter against the brake disk.
It is advantageous if the elastic element is configured as a spring system. In this case, spiral springs, for example, provide a suitable spring characteristic. In an alternative advantageous embodiment, it is possible to use not only a linear spring characteristic but also a progressive or stepped spring characteristic. In particular, soft springs, e.g. spring packs, are advantageous in order to ensure decoupling. Moreover, it is possible to combine a plurality of springs into an overall system, wherein the spring characteristics of the individual springs are incorporated in corresponding fashion into the characteristic of the overall system.
In an advantageous embodiment of the braking device, the elastic element is arranged between the spindle nut of the parking brake and the brake piston. In this case, the movement and/or the force of the electromechanical brake actuator of the parking brake is transmitted to the elastic element by the spindle nut in a direction axial with respect to the spindle of the parking brake.
It is advantageous if the braking device is designed to assume various states, wherein, in a first state, no force is transmitted to the brake disk, and, in a second state, a force, in particular a small force, is transmitted to the brake disk by means of the elastic element, and, in a third state, a force is transmitted to the brake disk by means of a geometric link.
This should be taken to mean that, in a first state of the braking device, there is decoupling between the actuator and the brake disk in respect of force transmission. This force transmission advantageously relates also to forces which arise due to component displacement, for example. By virtue of the decoupling, no forces are transmitted from the actuator to the brake disk, and also no forces are transmitted from the brake disk to the actuator.
In a third state, there is a geometric link—also referred to as a rigid or non-elastic link—between the brake disk and the actuator. By means of this geometric link, the elastic property of the elastic element is excluded. By means of this link, it is therefore likewise possible for forces and movements to be transmitted; however, transmission takes place without a significant reduction in the magnitude of the force or the movement. This link can be made indirectly by means of a plurality of components. The geometric link can be regarded as non-elastic, by virtue of direct contact between two substantially non-elastic components for example.
Even in the presence of an elastic component, a geometric link can be formed by bridging the elastic component—e.g. by means of a geometric configuration of components adjacent to the elastic component. It is advantageous if the geometric link is formed by means of positive connection of non-elastic components, e.g. a spindle nut and a brake piston. This should be taken to mean that the spindle nut and the brake piston have direct contact for the transmission of a movement or a force in the axial direction.
A geometric link can also arise from an elastic component if the elastic component is connected in such a way that it loses its elastic properties, e.g. through block formation by the elastic component. “Block formation” should be taken to mean that a further deformation—and hence further elasticity—is no longer possible, by virtue especially of geometrical restrictions. By way of example, attention should be drawn to a spiral spring, which forms a block or is moved so as to form a block as soon as the spring turns are resting upon one another and therefore no further spring property is effective.
It is advantageous, however, if, in a second state, which is passed through between the first and the third state, partial transmission of forces between the actuator and the brake disk is made possible by virtue of the elastic element, in particular by means of a soft spring. It is advantageous if the elastic element is designed in such a way that transmission of a movement and/or force, in particular an axial movement and/or force, from the parking brake to the brake disk is made possible, and transmission of a movement and/or force, in particular an axial movement and/or force, from the brake disk to the parking brake is prevented. It is thereby possible to avoid negative influencing—e.g. mechanical loading of the parking brake or even driver discomfort due to transmitted braking runout or noise. At the same time, however, preparation of a braking process, in the sense of a “brake prefill” is made possible and hence a safety-relevant function is provided.
This should be taken to mean that a force transmitted between the parking brake and the brake piston by means of the elastic element is sufficient to transmit a movement and/or a force of the parking brake to the brake piston and/or the brake pad, in particular in order to place the brake pad against the brake disk by means of brake piston and/or in order to allow pre-filling of a hydraulic chamber in the brake cylinder housing. Moreover, the transmitted force is small enough to avoid transmission of a movement and/or a force of the brake pad and/or of the brake piston to the parking brake, in particular in order to avoid transmission of braking runout of a brake disk to the parking brake by means of the brake pad and of the brake piston.
A method for controlling a braking device for a motor vehicle is furthermore provided, which device contains at least one brake actuator, wherein the braking device has at least one brake pad and a brake disk, and wherein the brake pad is placed against the brake disk in such a way that no braking force or only a small braking force acting on the motor vehicle is built up. According to the disclosure, this is characterized in that the brake pad is placed against the brake disk by means of an elastic element.
“Placing against” should be taken to mean that a release clearance between the brake pad or brake pads and the brake disk is substantially overcome. In particular, it is envisaged that the release clearance is completely overcome and the brake pad or brake pads actually rest against the brake disk. Here, both brake pads, for example, are centered and placed against the brake disk. According to the disclosure, a brake of the vehicle—to be more precise a brake actuator of a brake of the vehicle—can place the brake pad against the brake disk by means of an elastic element situated between the brake actuator and the brake disk without, however, building up a braking force or leading to any other effect, in particular force-related effect, between the brake disk and the brake actuator.
Here, it is advantageous if the method includes a method for controlling a parking brake and a method for controlling a service brake. Here, it is advantageous if the control of the braking device applies both to the control of the actuator and also, for example, to the control of valves in a hydraulic system. It is advantageous if the placing of the brake pads is accomplished by control of a parking brake.
It is advantageous if the method is started when a defined operating state of the motor vehicle is present. Such an operating state can be determined on the basis of data detected by means of sensors, for example. Such data relates, for example, to environmental data (e.g. detected pedestrians) and/or vehicle data (e.g. presence of acceleration/deceleration) and/or data on the driver (e.g. distraction). A potential emergency braking situation can represent one such operating situation, for example. Equally, however, the method can also advantageously be used continuously when rapid reactions are required, e.g. during an autonomous driving maneuver.
It is advantageous if, in the context of the method, the parking brake is controlled by means of path control. Path control is used to define a path which the actuator travels as it applies the parking brake. During the path control of the parking brake, at least one total release path and a force release path are taken into account, wherein the force release path takes account of a caliper stiffness and a clamping force, wherein, in particular, the caliper stiffness is determined in the context of the last parking brake operation performed, and wherein, in particular, the clamping force corresponds to the last clamping force set.
Caliper stiffness can also advantageously be measured during several locking operations and averaged in a suitable way. Force buildup between a first force (e.g. 0 kN on a fully released brake) and a defined second force (e.g. 12 kN) is described as delta F. This gives delta F=caliper stiffness*force release path. With a caliper stiffness of, for example, 30 kN/mm, a force release path of 0.4 mm is obtained. The total release path is always set to the same value, for example. Assuming a total release path s2 of, for example, 1.4 mm, an idle path of mm is obtained, wherein the difference between the total release path and the force release path defines the idle path (idle path=total release path−force release path).
Owing to possible disturbance variables, e.g. thermal relaxation, further tightening, locking under pressure or tolerances in the measurement system, there may be different effects: a path which is too short may be set by means of the path control system, for example. In this case, too short means that the brake piston is pushed at least partially out of the housing, wherein actual contact between the brake pads and the brake disk is not assured. By virtue of the path control and the defined path length traveled, however, it is nevertheless possible to assume that the release clearance has been substantially overcome. As an alternative, force estimation can be employed to detect actual contact. Moreover, a path which is too long can possibly be set during path control. Since the spring comes into action in this case, such an effect is relatively uncritical. However, provision is advantageously made to keep a sufficient spring operating path in reserve for such a case.
A service braking operation has at least two phases, wherein, in a first phase of the service braking operation, no force is transmitted from the service brake to the brake disk and, in a second phase of the service braking operation, a force is transmitted from the actuator of the service brake to the brake disk. A force is transmitted hydraulically, for example. In particular, the transmitted force produces a frictional force and/or a clamping force between the brake pad and the brake disk, which leads in turn to a braking force and/or a holding force acting on the motor vehicle. Here, the first and second phases describe a time sequence of a braking operation. It is advantageous if the first phase of the service braking operation is shortened, in particular eliminated, by means of the control of the parking brake.
A parking brake operation can furthermore have at least three phases, wherein, in a first phase of the parking brake operation, no force is transmitted from the parking brake to the brake disk and, in a second phase of the parking brake operation, a force is transmitted from the parking brake to the brake disk by means of the elastic element, and, in a third phase of the parking brake operation, a force is transmitted from the parking brake to the brake disk by means of a geometric link. A geometric link is taken to mean a rigid, i.e. non-elastic, link between at least two components. It is advantageous if the parking brake is operated in the first and second phases of the parking brake operation in order to shorten, in particular to eliminate, the first phase of the service brake operation. A force can be transmitted by virtue of the displacement of a component for example, e.g. the spindle nut, and can arise by virtue of a deformation of a component, e.g. the elastic element.
It is advantageous if the method for controlling the braking device has at least 4 steps, wherein, in a first step, the parking brake is activated, and, in a second step, the elastic element is placed against the brake piston by virtue of a change in the parking brake brought about by the activation, in particular a movement of an electromechanical device, and, in a third step, the elastic element is put under stress in such a way, by virtue of a further change in the parking brake brought about by the activation, in particular a further movement of an electromechanical device, that the force which arises moves the brake piston, and, in a fourth step, a further change in the parking brake, in particular a further movement of an electromechanical device, is prevented. It is advantageous if the prevention of the further movement in the fourth step takes place as soon as the brake pad moved by the brake piston has been placed or is deemed to have been placed against the brake disk. Path control can be used for this purpose. The described steps of the method also describe a time sequence.
It is advantageous if the method for controlling the braking device has a further, fifth step, in which the elastic element no longer has any elastic properties by virtue of a further change in the parking brake brought about by the activation, in particular a further movement of an electromechanical device. For this purpose, the elastic system is bridged. For example, a spring is moved so as to form a block or some other geometric, in particular positive, link between further components, e.g. the spindle nut and the brake piston, is achieved by means of the movement.
The method for controlling the braking device is advantageously distinguished by the fact that activation of control of the parking brake is based on a request by a specific driving function, and/or activation of control of the parking brake takes place automatically in a specific driving situation, and/or deactivation of the first operating mode of the parking brake takes place automatically when a specific driving function ceases, and/or deactivation of the first operating mode of the parking brake takes place automatically when a specific driving situation ceases. Activation and/or deactivation controlled by the driver can furthermore advantageously be implemented.
A control unit is furthermore provided, which is designed, i.e. set up, and has means to carry out the method described.
Is furthermore advantageously envisaged that the parking brake is a “motor on caliper” brake system.